Chemicals of interest have been tested for mutagenicity using a number of Salmonella tester strains. The chemicals studied, or being studied, were sodium bisulfite (SB), a series of nitrosamines, 2- and 4-acetylaminofluorene (2AAF and 4AAF), benzo(a)pyrene (BaP), pyrene, and a series of N-substituted phenanthreneimines. SB, an antioxidant, has been shown to be mutagenic at low pH, and the mutagenicity was bacterial strain-specific. The carcinogen-noncarcinogen pairs, 2AAF and 4AAF, and BaP and pyrene, were tested both in vitro and in the intrasanguineous host-mediated assay (HMA). All 4 chemicals are mutagenic in vitro, and the carcinogens (2AAF and BaP) gave consistently higher responses than the noncarcinogens (4AAF and pyrene). All 4 chemicals failed to induce significant mutagenic responses in the host-mediated assay. The cyclic nitrosamines are being tested in the HMA to determine if their pattern of organ-specific carcinogenicity can be predicted by any organ-specific mutagenicity detected in the HMA. The mutagenicity of the phenanthreneimines correlated with the electron-accepting or -donating ability of the substituent group, and inversely with their alkylating ability.